The objective is to determine the crystal structure of Pisum sativum Agglutinin (PSA), a mitogenic lectin isolated from green peas. This lectin has a molecular weight of about 49,000, it is Ca ions and Mn ions dependent, and it has an amino acid sequence that is partially homologous with that of the jack bean lectin, Concanavalin A (Con A). The crystal structure will be determined by techniques of protein crystallography, using multiple-isomorphous replacement methods. The structure will be compared with that of Con A. The results of these studies will be used to understand the structural features that are of principal importance in protein-carbohydrate interactions.